Indigo is a dark blue crystalline powder that sublimes at 390-392° C. (734-738° F.). It is insoluble in water, alcohols, and ethers and is soluble in DMSO, chloroform, nitrobenzene, and sulfuric acid (conc). The chemical formula of indigo is C16H10N2O2 and its IUPAC name is (2E)-2-(3-oxo-1H-indol-2-ylidene)-1H-indol-3-one.

Indigo is one of the first dyes to be used by humans for textile dyeing and printing. The compound has been detected on fabrics as old as 6000 years. Indigo is characteristically associated with its distinctive blue color. Indigo was originally extracted from the leaves of certain types of indigo plants, and was highly valued prior to the development of synthetic dyes since natural blue dyes are rare. The first laboratory synthesis of indigo was reported in 1878. By 1897, a commercially feasible manufacturing process was developed that largely replaced the use of naturally-sourced indigo. It is estimated that in 2013, 67,000 tons of synthetic indigo were produced worldwide.
Derivatives of indigo have also been used to obtain various colors. For example, thioindigo (NH groups replaced with S) is deep red; 6,6′-dibromoindigo is known as Tyrian purple; 5,5′,7,7′-tetrabromoindigo is known as Ciba blue; and sulfonation of indigo results in blue-green derivative known as “indigo carmine”. Indigo derivatives are employed as colorants in the food, pharmaceuticals, and cosmetics industries.
Indigo itself is problematic in fiber dyeing processes because it is insoluble in water. Commercially, the compound is dissolved in water by reduction to leucoindigo, also referred to as “indigo white”.
Changes in both color and solubility upon reduction are attributable to the loss of planarity of the molecule. The originally developed indigo dyeing process involves immersing cotton fibers in an alkaline dyebath including indigo and a reducing agent; the reducing agent reduces the indigo to leucoindigo in the bath. Upon removing the fabric from the dyebath, leucoindigo quickly oxidizes, reverting to the insoluble, intensely colored indigo. The solubility properties of indigo and the resulting severity of chemical conditions required to use it in aqueous vat dyeing processes historically limited its broader use.
N,N′-diacetyl indigo is a known derivative of indigo and is represented by the following formula:
The compound was reported at least as early as 1891 by Liebermann and Dickhutt, Chem. Ber. 1891, 24, 4130, further as reported in Journal of the Society of Dyers and Colorists, 1892, 8, 93. N,N′-diacetyl indigo is an air-stable, brick-red, particulate solid at 20°/1 atm. It is insoluble in water and obtains hues ranging from bright red to pink when dissolved in organic solvents. Isomerization, including photoisomerization, from trans to cis provides a photochromic shift to yellow hues.
N,N′-diacetyl indigo has been used as a disperse dye, employing temperatures between 180° C. and 220° C. to achieve colors ranging from blues to bright purples when combined with polyester or cellulose acetate fibers and fabrics.
Due to π-conjugation, indigo derivatives including N,N′-diacetyl indigo have been employed as organic semiconductors and functionalized with long-chain alkyl groups to form liquid crystalline morphologies. N,N′-diacetyl indigo has also been used in a direct positive bleach-out photographic system (exemplary is U.S. Pat. No. 3,595,659); in photoreactive optical elements for computer memory systems (exemplary are U.S. Pat. Nos. 4,288,861 and 4,459,682); and as means for storing solar energy (exemplary is U.S. Pat. No. 4,449,516).
In recent years, a return to natural sources of colorants has been driven by the desire to avoid industrial syntheses that externalize environmental pollutants. As a result, there is a renewed need for fiber dyeing methodologies that are useful in conjunction with naturally extracted plant-based colorants such as indigo, or simple derivatives thereof. There is a need for fiber dyeing methodologies that employ aqueous systems and reduce the amount of harmful chemicals used in the dyeing process. There is a need to broaden the toolkit of the fiber colorist using natural products and derivatives thereof as well as environmentally benign compounds and methods.